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Abstract:

This invention concerns a method and devices for calibrating a partial
discharge measuring device and for locating faults on cables. In the
method, calibration signals, which can include a band-limited white
noise, are used with a periodically repeated signal course. By averaging
over a predetermined period duration (T) of the calibration signal, it is
possible, in the case of a partial discharge measurement, to recalibrate
the measuring device continuously during the measurement, and
additionally on cables to determine the fault location with great
precision.

Claims:

1. A method of calibrating a partial discharge measuring device,
comprising: generating a calibration signal having a predetermined period
duration using a calibration signal generator, measuring a measuring
signal on a test object, and averaging the measuring signal using the
predetermined period duration of the calibration signal.

2. A method according to claim 1, wherein the calibration signal has a
periodically repeated signal course with the predetermined period
duration, the calibration signal is injected into the test object, in the
averaging, multiple measuring signal sections of the measuring signal,
each with the predetermined period duration, are averaged to an averaged
measuring signal section with the predetermined period duration, and the
partial discharge measuring device is calibrated on the basis of the
averaged measuring signal section.

3. A method according to claim 1, wherein the calibration signal is
injected into the test object while a voltage from a test voltage source
is applied to the test object.

4. A method according to claim 1, wherein the calibration signal includes
a band-limited white noise.

5. A method according to claim 4, wherein the white noise is generated by
means of a binary pseudo-random sequence.

6. A method according to claim 1, wherein the calibration signal includes
multiple discrete frequencies.

7. A method according to claim 1, wherein the partial discharge measuring
device is calibrated with an applied test voltage.

8. A method according to claim 1, wherein the measuring signal is carried
in at least two measurement paths, in a first measurement path of the at
least two measurement paths the measuring signal being averaged with the
predetermined period duration, and the calibration signal being measured,
and in a second measurement path of the at least two measurement paths
the partial discharge being measured.

9. A method according to claim 1, wherein a frequency range of a
band-pass filter is changed when a test voltage source is switched on,
and wherein the partial discharge measuring device is recalibrated
without the test voltage source being switched off.

10. A method according to claim 1, wherein a frequency band of a
band-pass filter is chosen so that frequencies of interfering signals in
the measuring signal in the chosen frequency band have essentially
smaller amplitudes compared with the corresponding amplitudes of the
calibration signal.

11. A method according to claim 1, wherein the predetermined period
duration in the partial discharge measuring device is known and set by
means of a PLL.

12. A method of locating faults on a cable, comprising the steps:
applying a signal to the cable, determining a group delay time of the
signal along the cable, determining a fault location on the basis of a
ratio between the group delay time and a delay time of partial discharge
pulses which occur on the cable.

13. A method according to claim 12, wherein the delay time measurements
are carried out by means of auto-correlation.

14. A method according to claim 12, wherein: the applied signal includes
a broadband signal which is repeated with a predetermined period
duration, and the group delay time for a frequency range in which a
partial discharge measurement is carried out is determined.

15. A method according to claim 14, wherein the applied signal is used to
calibrate the partial discharge measurement.

16. A method according to claim 12, wherein the applied signal includes a
band-limited white noise.

17. A method according to claim 12, wherein the signal is applied while a
test voltage is applied to the cable.

18. A method according to claim 12, wherein the cable is a power cable.

19. A partial discharge measuring system, including: a partial discharge
measuring device, which is designed for measuring a measuring signal on a
test object, and a calibration signal generator, which is designed for
generating a calibration signal, wherein the calibration signal has a
predetermined period duration, and wherein the partial discharge
measuring device is designed to use the predetermined period duration for
averaging the measuring signal.

20. A cable fault location system for locating faults on a cable,
including: a signal generating device to generate and apply a signal to
the cable, and a measuring system which is designed to determine a group
delay time of the signal on the cable, and to determine a fault location
on the basis of a ratio between the group delay time and a delay time of
partial discharge pulses which occur on the cable.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the benefit of European Patent Application
No. EP 10001913.2, filed on Feb. 24, 2010, the disclosure of which is
incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] This invention concerns a method of calibrating a partial discharge
measuring device, a method of locating a flaw on a cable by analysing
partial discharges, and corresponding systems.

[0003] To measure so-called partial discharges (PD) according to Standard
IEC 60270, calibrating the measuring device before the actual measurement
is prescribed and necessary. For calibration, a calibrating device is
usually connected between the test object and earth, and feeds simulated
partial discharge pulses of known charge into the test object. The
partial discharge measuring device is set to a frequency band with low
environmental interference levels, and is adjusted for this frequency
band so that it displays precisely the charge which is fed in by the
calibrating device. If the width or position of the frequency band
changes, recalibration is usually required. The pulse form for partial
discharge calibration is prescribed in Standard IEC 60270, and comes
close to a Dirac pulse, i.e. it involves short pulses of high amplitude
with a broad frequency spectrum. The pulses are repeated at short
intervals, to obtain a regular display on the measuring device. The
partial discharge calibration signal can therefore be considered as a
periodic signal, and a corresponding crest factor, which gives the ratio
of peak values to root mean square values, can be calculated. In the case
of the calibration signals described above, with pulse shape and high
amplitude, the result is very high values for the crest factor. This
means that the partial discharge measuring device to be calibrated must
have a measuring range with high dynamics and high precision.

[0004] The measuring signal is usually extracted using coupling capacitors
which are attached for the measurement, or using capacitances which are
inherently present in the system, e.g. capacitive implementations of
transformers or circuit breakers or capacitive coatings which are present
between the shield and core of power cables. The calibrating device which
is connected for calibration is usually removed after calibration, since
it cannot withstand the test voltage. A test voltage is then applied to
the test object, and the actual partial discharge measurement is carried
out.

[0005] In the case of a partial discharge measurement on a cable, the
distance from a measuring device, which is usually at a cable end, to an
imperfection at a fault location in the cable can usually be determined
indirectly by determining signal transit times of interfering pulses. The
fault location is usually determined by applying a test voltage to the
cable. At the fault location, partial discharge pulses then occur. The
pulses have a certain transit time until they reach the measuring device.
Because, at the cable end, for each pulse there is an echo, the
difference of the transit times between pulse and echo can be measured.
If the fault location is very near the end of the cable, the transit time
difference between pulse and echo is small. A large transit time
difference indicates a fault near the measuring point. To be able to
calculate precisely where the fault location is, the speed at which an
interfering pulse is propagated in the cable must be known. This can be
determined by means of a calibration pulse, which is preferably fed into
the near cable end at the measuring device and reflected at the far end
of the cable. The transit time is calculated, and the speed of the pulse
propagation is determined from it. The speed depends on the temperature
of the cable and the measurement frequency window under consideration.

[0006] From the above description, it becomes clear that calibration of a
partial discharge measuring device according to the prior art is very
resource-intensive, since the partial discharge measuring device must be
recalibrated every time the frequency band is changed or the temperature
of the test object changes. For calibration, each time, the test voltage
must be switched off, the calibrating device must be connected, and after
calibration and before the actual partial discharge measurement it must
be removed again. Additionally, in the case of the calibration method
according to the prior art, the requirements regarding the dynamics on
measuring circuits of the partial discharge measuring device are high.
The same applies to current methods of locating faults on cables.

SUMMARY

[0007] The object of this invention is therefore to provide appropriate
simplified and improved methods.

[0008] According to this invention, this object is achieved by a method of
calibrating a partial discharge measuring device according to claim 1, a
method of locating faults on a cable according to claim 12, a partial
discharge measuring system according to claim 19 and a cable fault
location system according to claim 21. The dependent claims define
preferred and advantageous embodiments of the invention.

[0009] According to this invention, a method of calibrating a partial
discharge measuring device is provided. In the method, a periodic
calibration signal with a periodically repeated signal course of
predetermined period duration is generated. The periodic calibration
signal is, for example, injected into a test object, and the partial
discharge measuring device captures a measuring signal. The measuring
signal can include multiple measuring signal sections, each with the
predetermined period duration of the calibration signal. The
predetermined period duration is used for averaging the measuring signal.
For example, by averaging multiple measuring signal sections, each with
the predetermined period duration, an averaged measuring signal section
is determined. On the basis of this averaged measuring signal section,
the partial discharge measuring device can then be calibrated. To
determine the averaged measuring signal section, for example, many
thousand measuring signal sections, which are captured in succession at
the test object, can be used, while the periodic calibration signal is
injected into the test object. Since a periodic calibration signal with a
defined amplitude course is fed in, this calibration signal, compared
with other signals in the test object, e.g. a partial discharge measuring
signal or an interfering signal, can be very small, since to calibrate
the partial discharge measuring device, the fact that the calibration
signal is periodic, so that by averaging over very many periods the
calibration signal can be captured precisely, despite the low level, can
be exploited.

[0010] These and other objects and features of the present invention will
become more fully apparent from the following description and appended
claims, or may be learned by the practice of the invention as set forth
hereinafter.

BRIEF DESCRIPTION OF DRAWINGS

[0011] To further clarify the above and other advantages and features of
the present invention, a more particular description of the invention
will be rendered by reference to specific embodiments thereof which are
illustrated in the appended drawings. It is appreciated that these
drawings depict only illustrated embodiments of the invention and are
therefore not to be considered limiting of its scope. The invention will
be described and explained with additional specificity and detail through
the use of the accompanying drawings in which:

[0012] FIG. 1 shows schematically how, using an embodiment of a method
according to the invention, interference in one measuring signal section
is suppressed by averaging multiple measuring signal sections.

[0013]FIG. 2 shows a spectrum of a calibration signal formed from
discrete frequencies.

[0014]FIG. 3 shows a partial discharge measuring system according to an
embodiment of this invention.

[0015]FIG. 4 shows a cable fault location system according to an
embodiment of this invention.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

[0016] According to an embodiment, in the method a test voltage is applied
to the test object while the periodic calibration signal is injected. In
this way, a large time saving can be achieved in the execution of the
partial discharge measurement, and the whole course of the partial
discharge measurement can be simplified. In the case of a change of the
frequency band in the partial discharge measuring device, e.g. a shift of
a filter frequency of the partial discharge measuring device, switching
the test voltage off and on is thus unnecessary, and instead the partial
discharge measuring device can be calibrated with the test voltage
switched on. The partial discharge measuring device can also be
calibrated continuously while the test voltage is applied to the test
object. Other possible changes in the system, e.g. the temperature of the
test object, can thus be continuously detected and calibrated away. The
amplitude of the periodic calibration signal is preferably chosen to be
less than the expected amplitudes of partial discharges in the test
object. Thus the calibration signal does not significantly affect the
measurement result of the actual partial discharge pulses. Additionally,
the smaller calibration signals can be connected to a test voltage
potential via coupling capacitors more easily than large calibration
signals or calibration signals with high dynamics.

[0017] According to a further embodiment, the signal course of the
calibration signal includes a band-limited white noise. The band-limited
white noise can, for example, be a white noise which is limited by a
low-pass filter. White noise has a significantly smaller crest factor
compared with repeated pulses, and also has the property of covering the
whole spectrum within the limited frequency band.

[0018] According to a further embodiment, the white noise can be generated
by means of a binary pseudo-random sequence, which is preferably then
low-pass filtered.

[0019] According to a further embodiment, the periodic calibration signal
can include a frequency mixture of multiple discrete frequencies. For
example, the frequencies can be distributed in the spectrum so that the
whole possible range of the partial discharge measurement is covered. By
suitable choice of the discrete frequencies, periodicity of the whole
sequence is achieved. The periodicity of the whole frequency corresponds
to the greatest common denominator of the discrete frequencies. If a 2
kHz and a 3 kHz signal are used as the discrete frequencies, the result
is a frequency for the calibration signal of 1 kHz. In a further example,
if a 2 kHz and a 3.1 kHz signal are used as the discrete frequencies, the
result is a periodic calibration signal with a frequency of 100 Hz. By
optimising the phase position between the discrete frequencies, a further
reduction of the crest factor can be achieved.

[0020] Since the injected periodic calibration signal can be small
compared with a partial discharge measuring signal corresponding to a
partial discharge in the test object, the calibration signal can be
applied continuously during the actual partial discharge measurement,
without the partial discharge measurement being significantly influenced
by it. The permanent presence of the calibration signal during the
partial discharge measurement can be used to track the calibration
continuously during the partial discharge measurement, and to readjust
the partial discharge measuring device so that correct results are always
displayed. This is advantageous, in particular, if the frequency band is
shifted or the bandwidth is changed, but also if recalibration is
necessary because of changes in the test object or changes in the partial
discharge measuring device.

[0021] According to a further embodiment, the measuring signal is
separated into at least two measurement paths according to, for example,
a band-pass filter. In a first measurement path, averaging over the
predetermined period duration is carried out, and then a partial
discharge measurement is carried out. Since true partial discharges and
interfering signals do not have precisely the predetermined period
duration, they are invisible in this measurement path, and only the
calibration signal is measured here. In a second measurement path, a
traditional partial discharge measurement is carried out. If the
calibration signal is chosen to be so small that it causes no significant
change in the second measurement path, a partial discharge can be
measured and the system can be calibrated simultaneously.

[0022] According to a further embodiment, it is possible to change a
band-pass filter with the test voltage source switched on, and to carry
out recalibration without switching the test voltage source off. This is
possible because of the injection of the calibration signal at test
voltage level and the low dynamics which are necessary when white noise
is used. Alternatively, instead of white noise, a different periodic
signal can be used.

[0023] According to a further embodiment of this invention, a frequency
band of the periodic calibration signal is chosen so that frequencies of
an interfering signal in the measuring signal have, in the chosen
frequency band, smaller amplitudes than corresponding amplitudes of the
periodic calibration signal. For example, a broadband calibration signal
can be applied during the measurement, and then a range in the frequency
band in which an interference level is small compared with the
calibration signal can be determined. The calibration signal can
preferably have a constant level over the frequency. The frequency
response of the calibration signal is compared with the frequency
response of the interference level, and a range with a large gap between
calibration signal and interference level is determined. This range then
also represents an optimal range of a frequency band for partial
discharge measurement. The frequency band can be set using a band-pass
filter, for example.

[0024] In the described method, the coherence between the calibration
signal and the part of the partial discharge measuring device which
measures the calibration signal is exploited. To ensure coherence, the
partial discharge measuring device can be aligned with the calibration
signal with respect to its phase position. This can be achieved, for
example, using a so-called phase locked loop (PLL) in the partial
discharge measuring device.

[0025] A further method of this invention makes it possible, if the test
object includes an electrical cable with a flaw, to determine a fault
location on the cable. For this purpose, a periodic calibration signal
with a predetermined period duration is injected into the cable. The
measuring signal is normally measured and captured near the injection,
preferably at one end of the cable, together with partial discharge
pulses which occur on the cable. As described above, an averaged
measuring signal section is determined by averaging multiple measuring
signal sections with the predetermined period duration of the measuring
signal. In this way, because of the coherence of calibration signal and
averaging, interfering signals can be suppressed arbitrarily well. For
the averaged measuring signal section, a transit time between the
periodic calibration signal and an echo of the periodic calibration
signal is determined. Using the transit time, the distance to a cable
position where the echo was generated is determined. The cable position
where the echo is generated is preferably the end of the cable, but could
also be an imperfection such as a break or short circuit of the cable. In
this way, if the length is known, the transit time of pulses in a
specified frequency band can be determined very precisely, or if the
transit time in the case of at least one frequency is known, the length
of the cable can be determined. If partial discharge pulses which occur
on the cable are related in the same way without averaging, the fault
location can be determined very precisely, and a new determination of the
transit time because of temperature changes or changed measurement
frequency bands is possible without having to switch off the test
voltage. It is also possible, if the transit time on a cable is known at
only one frequency, to determine the cable length at this frequency, and
then to determine the group delay time on the cable at other frequencies,
which are more suitable for partial discharge measurement.

[0026] According to a further embodiment, the calibration signal is a
broadband periodic signal with the predetermined period duration, and the
group delay time is determined in each case for a specified frequency
range. For this purpose, averaging takes place over the predetermined
period duration according to, for example, a band-pass filter, and then
the time between the calibration signal and the echo from the cable end
is determined. The now known group delay time is taken into account in
the measurement of the transit time of the interfering pulses, and the
measurement therefore becomes much more precise.

[0027] A further embodiment uses an auto-correlation method for transit
time measurement.

[0028] According to a further embodiment, the partial discharge pulses
which occur at the imperfection of the cable are not only used to
determine the fault location, but also captured quantitatively. The
calibration signal which is used to determine the transit time of the
signal if the cable length is known, or the cable length if the transit
time is known, can additionally be used for calibration, as described
above.

[0029] According to a further embodiment, the calibration signal includes
band-limited white noise.

[0030] According to a further embodiment, the calibration signal for
determining the group delay time, cable length and calibration of the
partial discharge system is injected even if test voltage is applied.

[0031] According to a further embodiment, the test object is a power
cable. In particular with this equipment, the method of partial discharge
measurement is usual, and can be simplified specially greatly by this
invention.

[0032] According to this invention, a partial discharge measuring system
is also provided. The partial discharge measuring system includes a
partial discharge measuring device, which is designed for measuring a
measuring signal on a test object, and a calibration signal generator,
which is designed for generating a calibration signal. The calibration
signal has a predetermined period duration. The partial discharge
measuring device is designed to use the predetermined period duration for
averaging the measuring signal.

[0033] According to an embodiment, the partial discharge measuring system
is designed to carry out the method described above for calibrating the
partial discharge measuring device, and therefore also includes the
advantages described above.

[0034] According to this invention, a cable fault location system for
locating faults on a cable is also provided. The cable fault location
system includes a signal generating device to generate and inject a
signal onto the cable, and a measuring system which is designed to
determine a group delay time of the signal on the cable, and to determine
a fault location on the basis of a ratio between the group delay time and
a transit time of partial discharge pulses which occur on the cable.

[0035] According to an embodiment, the cable fault location system is
designed to carry out the method described above for locating faults on
the cable, and therefore also includes the advantages described above.

[0036] FIG. 1 shows a signal course 6 with a predetermined period duration
T, which is repeated periodically and is injected into a test object as a
calibration signal to calibrate a partial discharge measuring device.
FIG. 1 also shows a measuring signal section 1, which shows a
corresponding section of a measuring signal which is captured with the
partial discharge measuring device on the test object while the signal
course 6 is injected. The measuring signal section 1 thus includes a
total signal from the signal course 6 and further signals on the test
object. The further signals are mainly overlaid interfering signals,
which are injected from the environment of the test object into the test
object. The totality of the further signals can be considered as an
interfering signal in relation to calibration of the partial discharge
measuring device. Consequently, the measuring signal section 1 includes a
total signal of the signal course 6 and the interfering signal.

[0037] FIG. 1 also shows various averaged measuring signal sections 2-5.
An averaged measuring signal section 2-5 is formed by averaging multiple
different measuring signal sections 1. Each measuring signal section 1
which is used for averaging has the predetermined period duration T of
the signal course 6, and is captured phase-locked to the periodically
repeated signal course 6. Consequently, every measuring signal section 1
which is used for averaging has the identical signal course 6, with
identical phase position, and additionally an individual interfering
signal, which from the point of view of the periodic calibration signal,
i.e. with respect to the period duration and phase position of the signal
course 6, can be seen as random. Assuming that the random interfering
signals in the captured measuring signal sections 1 are unbiased, since
for example they are taken from the test object via a capacitive
coupling, the result is that the more the measuring signal sections 1 are
averaged, the more the averaged measuring signal sections 2-5 approach
the signal course 6. For example, the averaged measuring signal section 2
was formed by averaging ten measuring signal sections 1. For example, the
averaged measuring signal section 3 was formed by averaging 50 measuring
signal sections 1. For example, the averaged measuring signal section 4
was formed by averaging 200 measuring signal sections 1, and finally the
averaged measuring signal section 5 was formed by averaging 1000
measuring signal sections 1. As can be seen in FIG. 1, the signal course
6, despite its small amplitude, can be filtered by averaging signals with
overlaid interfering signals with great amplitude. The signal courses of
FIG. 1 are shown as a voltage over time, for example.

[0038] As the signal course 6 for the periodic calibration signal, for
example a pseudo-randomly generated and low-pass filtered white noise or
a frequency mixture of discrete frequencies can be used. If a frequency
mixture of discrete frequencies is used, the frequencies are preferably
distributed in the frequency spectrum so that a total possible frequency
range of a partial discharge measurement is covered, to ensure
calibration of the partial discharge measuring device over the whole
possible range of the partial discharge measurement. Additionally, the
discrete frequencies can be chosen so that as few interference problems
with signals of a partial discharge measurement as possible occur. For
example, FIG. 2 shows a possible spectrum of discrete frequencies 10
which can be used to form the periodic calibration signal.

[0039]FIG. 3 shows a partial discharge measuring system for measuring
partial discharges in a test object 61, a so-called device under test.
The test object 61 is connected to earth potential via an earthing
arrangement 60. The partial discharge measuring system includes a partial
discharge measuring device 41, a calibration signal generator 31 and a
test voltage source 21. For example, the test object 61 can be a power
transformer or a power cable or any other power device with which partial
discharges can occur. The partial discharge measuring device 41 is
connected to the test object 61 via a coupling capacitor 42. The partial
discharge measuring device 41 is also connected to earth potential via an
earthing arrangement 40. The calibration signal generator 31 is connected
to the test object 61 via a coupling capacitor 32. The calibration signal
generator 31 is also connected to earth potential via an earthing
arrangement 30. The test voltage source 21 can connect and disconnect a
test voltage to the test object 61 via a switch 22, and is also connected
to earth potential via an earthing arrangement 20.

[0040] The calibration signal generator 31 generates a periodic
calibration signal with a periodically repeated signal course such as the
signal course 6 shown in FIG. 1. The signal course 6 has a predetermined
period duration T, which is provided by a time source 35 of the partial
discharge measuring system. Via the coupling capacitor 32, the periodic
calibration signal is injected into the test object 61. Via the coupling
capacitor 42, a measuring signal from the test object 61 is fed to the
partial discharge measuring device 41. The measuring signal includes
partial discharge pulses, which may occur in the test object 61 because
of the applied test voltage, interference which is injected externally,
and the calibration signal, as described in relation to FIG. 1. In the
partial discharge measuring device 41, as much interference as possible
is filtered away at the input of the partial discharge measuring device
41 by a band-pass filter 51 of the partial discharge measuring device 41.
The measurement path is then split. In one measurement path, using an
averager 50 of the partial discharge measuring device 41, the measuring
signal is averaged over the period duration T, which is provided by the
time source 35, and the calibration signal can be filtered cleanly of
interference and partial discharge pulses, even if its amplitude is
significantly below that of other signals. The filtered calibration
signal is measured like a normal partial discharge in a corresponding
measuring unit 44 of the partial discharge measuring device 41. In the
other measurement path, the actual partial discharge is measured in a
corresponding measuring unit 47 of the partial discharge measuring device
41. By the choice of a very small calibration signal compared with the
partial discharge, the calibration signal influences the actual partial
discharge measurement only insignificantly. With the aid of a partial
discharge arithmetic unit 43 of the partial discharge measuring device
41, the partial discharge measuring device 41 is calibrated on the basis
of the filtered calibration signal.

[0041] By connecting the calibration signal generator 31 via a coupling
capacitor 32, the test voltage source 21 can be switched on and
simultaneously calibrated, which represents a considerable advantage
compared with the prior art.

[0042] For the case that the test object 61 is a cable, FIG. 4 shows a
cable fault location and partial discharge measuring system, which as
well as the possibility of measuring the partial discharges on the cable
61 as described in FIG. 3, can also determine the location or locations
at which partial discharges occur in the cable 61. Components which have
already been described in relation to FIG. 3 have the same reference
symbols in FIG. 4, and are not described again.

[0043] Parallel to the two partial discharge measuring units 44 and 47 for
the calibration signal and true partial discharges respectively, transit
time measuring devices 45 and 46 are connected for time measurement.
Because high frequency signals along a cable 61 have a certain transit
time, and are also reflected at an open cable end 65, auto-correlation
functions can be determined for the signals. An auto-correlator 48 for
the calibration signal supplies time information to the time unit 45
corresponding to double the transit time of the signal from the
calibration source 31 along the cable 61. In this way, if the transit
time or propagation speed for a specified frequency is known, the cable
length can be determined, or if the cable length is known, the transit
time or propagation speed on the cable can be determined. The actual
partial discharge information has shorter time information, which is
determined by means of an auto-correlator 49 and the time measuring unit
46. The nearer an imperfection 62 and thus a partial discharge source is
to the cable end 65, the smaller is the gap between direct reception and
echo. Using the information from the time measuring units 45 and 46, the
arithmetic unit 43 can determine the fault location exactly.

[0072] The present invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by the foregoing
description. All changes which come within the meaning and range of
equivalency of the claims are to be embraced within their scope.